This invention relates to an aquarium heater and more particularly to a heater which can maintain its operation in a more accurate, efficient, and safe manner.
Aquarium heaters are generally utilized for maintaining a desired temperature level in aquarium tanks in order to provide a suitable environment for sustaining and propagating the fish contained in the tank. As a result, it is necessary to provide the heater with appropriate thermostatic means in order to sense the changing temperature in the tank and to thereby appropriately turn on and off the heater to maintain the temperature level as close as possible to the desired value. Normally, the aquarium heater is hung over the side of the tank by means of a housing, and the heating element is contained within a tube depending from the housing. Inside the tube is also contained an appropriate thermostat which senses the temperature. Typically, a bimetallic thermostatic switch is utilized, having a fixed and adjustable control member. The two control members are initially placed in appropriate contact position in accordance with the desired temperature level. As the surrounding temperature of the water changes, the bimetallic control members will move into and out of engagement with each other in order to open and close the circuit.
The heater is normally placed in the circuit and is turned on and off by means of the bimetallic switch. Usually, the heater is placed at the lower end of the tube and the bimetallic elements and other circuit members are contained in the upper part of the tube. The control of the temperature level is generally obtained by means of a control knob placed on the housing with indicia on the knob so that the desired temperature level can be selected externally of the aquarium tank.
In order to prevent arcing or pitting of the contacts, a capacitor is typically placed in parallel across the contact switch. Additionally, an indicator bulb, or the like, is generally included in parallel with the heating element to provide an indication when the heating element is turned on. Numerous heaters are described in the prior art. Typically, an aquarium heater is described in U.S. Pat. No. 3,731,058 which shows an aquarium heater of the type described and includes a unique heating coil arrangement which maintains uniform heating. Another prior art aquarium heater is described in U.S. Pat. No. 3,107,289 which again utilizes a basic structure similar to that described, and further includes two bimetallic switches in parallel in order to prevent freezing or sticking of the contact members of the first one of these switches.
While these and other aquarium heaters are generally in use, one of the problems with such heaters concerns the maintenance of the temperature level as close as possible to the desired value. The thermostat within the tube should monitor the temperature of the aquarium water and react to that temperature alone. However, since it is positioned in the tube adjacent to the heating element itself, it is also affected by such heating element. As a result, the heat reflected from the heating element is sensed by the thermostatic switch so that the switch may react to this increase in temperature and turn off the heating element, although actually, it is not the aquarium water that has risen in temperature, but it is only the effect from the heating element which has raised the temperature at the thermostatic switch. Therefore, in prior art heaters, there is more frequent turning on and off of the heater, and more difficulty in maintaining the temperature level of the aquarium water at the desired value. Additionally, the continued turning on and off of the bimetallic switch more than necessary, can cause premature carbonization and/or pitting of the switch contacts.
Additional problems with prior art devices concern the proper manufacture and maintenance of the device. Generally, the housing is formed of molded plastic which is manufactured to particular tolerances. The tube, typically of glass, is manufactured seperately and also has its own broad tolerances. As a result, the spacing between the tube and the housing can vary between different aquarium heaters. With prior art devices, little, if anything, is done to accommodate such variations and accordingly problems of excessive heat leakage, water seepage from aeration bubbles, poor efficiency, and difficulty in maintenance is often encountered because of such variations.
A further problem of prior art devices concerns the specific construction whereby the control chassis supporting the thermostatic elements is frequently utilized to lock the housing onto the tube. For example, in U.S. Pat. No. 3,107,289, there are locking members on the control chassis which extend upwardly into the housing and are utilized for locking the various parts of the housing onto the tube. As a result, should movement of the tube occur, as for example during shipping and/or actual use, the control chassis together with the thermostatic elements will be jarred and invariably moved from their preset position. Likewise, various electrical leads, capacitor, indicator bulb, and other circuit components can also be displaced as the housing portion and the tube are slightly moved relative to each other. Such displacement can frequently occur during actual utilization of the aquarium heater as it is being installed on the aquarium tank. During such installation, manipulation of the housing with relation to the tube always occurs. With prior art devices, such manipulation will effect the control chassis and accordingly the thermostatic elements, which can permanently damage the aquarium heater. Also, even if it does not damage the heater, it may modify the present positioning of the contact members forming the thermostatic switch, whereby the initial setting on the control knob will no longer be accurate to the desired temperature setting and not infrequently can harm the fish by too great a temperature deviation.
Yet other problems with prior art devices concern the method and apparatus utilized for controlling the setting of the temperature level at the desired value. The most common device utilized for such control is a screw. For example, in the aforementioned patent, there is utilized an adjustable screw rotated by a control knob which bears against a bridge arrangement interconnecting the adjustable contact with respect to the fixed contact. Such screw arrangement can fail during continued utilization, where the screw or opposing threads can become stripped. Also, the screw can lock the bridge arrangement into place whereby it cannot be raised, and the temperature level may be thereby held at a critically dangerous high level. The threaded metal or plastic rod utilized as the screw in prior art aquarium heaters is not perfectly straight and therefore wobbles or turns eccentrically, creating a non-linear temperature range. Therefore, when the screw is turned to raise or lower the temperature slightly, the wobble acts in reverse. The screw arrangement has been a difficult one to maintain as well as utilize in order to provide accurate and efficient temperature setting.